The present invention relates to a process for the preparation of saturated C3-C20-alcohols in which a liquid hydrogenation feed comprising at least one C3-C20-aldehyde is passed over a bed of a hydrogenation catalyst in the presence of a hydrogen-containing gas.
The catalytic hydrogenation of aldehydes in order to obtain alcohols is a process which has been carried out on an industrial scale for decades and in which a multiplicity of catalysts, which generally consist of elements from sub-groups VI to VIII and I of the Periodic Table, in particular of the elements chromium, manganese, iron, cobalt, nickel and/or copper, are employed. Such catalysts are described, for example, in DE-A 32 28 881, DE-A 26 28 987 and DE-A 24 45 303. The alcohols prepared by these processes find broad use, for example as solvents or as plasticizer alcohols.
In the hydrogenation, in particular at high hydrogenation temperatures, undesired side reactions, such as acetalization or aldolization, the Tischtschenko reaction or ether formation, occur in addition to the desired hydrogenation of the aldehyde to the alcohol. These side reactions result in a reduction in the product yield and require increased effort during purification of the hydrogenation product in order to obtain the relevant alcohol in the desired purity.
In order to avoid side reactions of this type, DE-A 26 28 897 recommends adding water to the hydrogenation feed. However, this measure has a number of disadvantages; for example, the energy necessary for purification of the resultant alcohols by distillation increases significantly.
Another possibility for reducing the formation of by-products comprises increasing the hydrogen pressure in the hydrogenation, which increases the rate of the hydrogenation reaction, while the reaction rate of the side reactions which are independent of the hydrogen pressure remains the same. Overall, the selectivity with respect to the desired hydrogenation product thus increases.
However, an increase in the hydrogen pressure is associated with high equipment complexity, since, for safety reasons, pressurized reactors with thicker walls must be used and further safety precautions have to be taken.
German patent 16 43 856 describes the hydrogenation of aldehydes by means of a copper- and/or nickel-containing catalyst whose surface has been adjusted to a pH of from 6 to 10 by treatment with alkali metal hydroxides. This publication is expressly directed to the use of the catalysts pretreated in this way in gas-phase hydrogenation. Their use in liquid-phase hydrogenation is only possible to a limited extent. The alkali metal hydroxide is usually washed out by the liquid hydrogenation feed or the liquid hydrogenation products and removed from the reaction system, and consequently the advantages of the surface treatment of the catalyst are only short term.
JP 172 838 A relates to the hydrogenation of C5- and higher aldehydes on a nickel/chromium catalyst in the presence of a tertiary aliphatic amine.
JP 171 447 A relates to the hydrogenation of C4-aldehydes to butanol on a nickel/chromium catalyst in the presence of a tertiary aliphatic amine. In both the last-mentioned processes, the added amine is separated off from the hydrogenation product by subsequent distillation and advantageously fed back into the hydrogenation. However, pure amine is not recovered in the distillation, but instead a mixture of the amine with so-called high boilers, i.e. the by-products which boil higher than the target alcohol and are formed in the hydrogenation of aldehydes, is obtained. The recycling of the amine/high boiler mixture requires that a ballast of high boilers is always circulated through the hydrogenation and distillation. Since, in order to avoid increases in concentration, a part of the high-boilers which corresponds to the formation rate of the high boilers must always be removed from the circuit, amine losses are unavoidable and represent an additional economic burden for the process.
WO 96/26173 describes a process for the purification of C3-C10-alcohols by distillation, where the distillation is carried out in the presence of an alkali metal hydroxide. This publication makes no mention of the addition of a salt-like base to a liquid hydrogenation feed.
It is an object of the present invention to indicate a process for the preparation of saturated alcohols from aldehydes by liquid-phase hydrogenation in which the formation of undesired by-products is suppressed, in particular at hydrogenation temperatures of 150xc2x0 C. or above, and which is free from the disadvantages of the known hydrogenation processes.
We have found that this object is achieved by a process in which a liquid hydrogenation feed comprising at least one C3-C20-aldehyde is passed over a bed of a hydrogenation catalyst in the presence of a hydrogen-containing gas, which comprises adding to the hydrogenation feed an amount, homogeneously soluble therein, of a salt-like base [M+]n [Anxe2x88x92], in which [M+] is an alkali metal ion or the equivalent of an alkali earth metal ion; [Anxe2x88x92] is an anion of an acid having a pKs value of greater than 2, and n is the valency of the anion.
The effect of the addition of base to the hydrogenation feed is that the side reactions outlined at the outset are substantially suppressed even at hydrogenation temperatures of 150xc2x0 C. or above, and very pure alcohols are obtained even at these hydrogenation temperatures.
The type of salt-like base used is generally not crucial so long as the salt-like base used is homogeneously soluble in the hydrogenation feed, at least in low concentration, and does not undergo any undesired side reactions with the aldehyde. Accordingly, a multiplicity of salt-like bases can successfully be employed in the process according to the invention.
The bases employed in accordance with the invention are salt-like, i.e. they are built up from cations and anions; they comprise at least one alkali metal or alkaline earth metal cation, such as lithium, sodium, potassium, magnesium or calcium ions, and a basic anion. The corresponding acid of the basic anion has a pKa value of greater than 2, preferably greater than 4, in particular greater than 8. The pKa value used for the characterization of the acid strength of the corresponding acid is the negative decimal logarithm of the dissociation constant of the acid in dilute aqueous solution. The pKa values of numerous acids have been tabulated and are given, for example, in CRC Handbook of Chemistry and Physics, 76th Edn., 1995, CRC Press; Organikum, various authors, 16th Edn., VEB Deutscher Verlag der Wissenschaften 1986, p. 138; Sykes P., Reaktionsmechanismen der Org. Chemie, 8th Edn. 1982, p. 307.
Suitable basic anions are hydroxide (14), carbonate (10.33), hydrogencarbonate (6.35), phosphate (12.35), amide (35), hydride (39); alkoxides, in particular C1-C4-alkoxides, such as methoxide (16), ethoxide, n- and isopropoxide and butoxide; phenoxide (10), carboxylates, such as acetate (4.76) or benzoate (4.21); carbanions, such as butyl (50), cyclopentadienyl or phenyl (40). The values in brackets indicate the pKS value of the respective corresponding acid. Besides the hydride ion itself, complex hydrides are also suitable; these can be regarded as adducts of the hydride ion and their basicity is essentially due to the, hydride ion, for example complex hydrides such as [BH4]xe2x88x92 or [BHR3]xe2x88x92 (where R=C1-C4-alkyl, for example s-butyl).
In general, hydroxide or carbonate is preferred.
Advantageous salt-like bases are, in particular, alkali metal hydroxides and/or carbonates, such as lithium carbonate, potassium carbonate, sodium carbonate, lithium hydroxide, sodium hydroxide and potassium hydroxide. In general, sodium hydroxide and/or potassium hydroxide are preferred. However, sodium alkoxides and/or potassium alkoxides, such as the methoxide or ethoxide, or the alkoxide of the alcohol which is the hydrogenation product of the aldehyde present in the hydrogenation feed can also be employed with particular advantage.
The salt-like bases are generally added to the hydrogenation feed in an amount which corresponds, on neutralization equivalent basis, to from 0.1 to 2000 ppm by weight, preferably from 0.1 to 1000 ppm by weight, in particular from 0.1 to 100 ppm by weight, particular preferably from 0.5 to 50 ppm by weight and in particular from 1 to 20 ppm by weight, based on the aldehyde present in the hydrogenation feed, of potassium hydroxide. In the case of monovalent basic anions, a molar amount of salt-like base which corresponds to the stated amount of potassium hydroxide is used, and in the case of divalent basic anions, half the molar amount is used. It is also possible to add mixtures of different bases.
Owing to the low salt-like base concentrations employed and the generally low price of these bases, recovery is not necessary or advantageous.
The liquid hydrogenation feed can consist of one or more undiluted aldehydes. However, the aldehydes are preferably employed as a solution in an inert diluent. Examples of suitable inert diluents are hydrocarbons, ethers, such as diethyl ether, or alcohols. The diluents are particularly preferably alcohols, in particular the alcohol which is the hydrogenation product of the aldehyde to be hydrogenated. In a preferred embodiment, a part amount of the hydrogenation product is recycled for this purpose and mixed with the aldehyde to be hydrogenated. If used, the inert diluent is preferably used in an amount of from 0.1 to 100 parts by weight, in particular from 1 to 50 parts by weight and particularly preferably from 5 to 20 parts by weight, based on one part by weight of aldehyde employed. If the hydrogenation is carried out adiabatically, i.e. with removal of the heat of reaction by the reaction product, the amount of inert diluent used is advantageously such that the temperature gradient over the bed of the granular catalyst does not exceed 40xc2x0 C. If, by contrast, the hydrogenation reactor is operated isothermally, the proportion of inert diluent in the hydrogenation feed can be selected virtually as desired.
The hydrogenation feed generally contains traces of water, for example in the order of from 1 ppm to 1% by weight, which have been introduced by the starting materials in the preceding synthesis steps or formed by condensation reactions. These traces of water are unimportant for the process according to the invention. On use of salt-like bases other than hydroxides, hydroxide ions are formed therefrom by transprotonation and/or hydrolysis, which is in accordance with the invention.
The process according to the invention can be carried out either batchwise or continuously, for example with the aid of tubular reactors or reactor cascades. The catalyst bed generally rests on a suitable retention element in the reactor. The hydrogenation reactor can be operated either by the pool or trickle method. The process according to the invention is preferably carried out in a reactor cascade, in particular a cascade comprising two to five reactors.
The salt-like base can be added in solid or dissolved form; it is preferably added in the form of its solution in water or an alcohol, in particular the alcohol which is the hydrogenation product of the aldehyde present in the hydrogenation feed. For example, from 1 to 40 percent by weight solutions are suitable. The base and the hydrogenation feed can be introduced into the hydrogenation reactor separately from one another, with the mixture of base and hydrogenation feed forming in situ in the reactor. In particular in the case of the continuous procedure, however, a preformed mixture of based and hydrogenation feed is preferably passed into the reactor. If, as in a preferred embodiment, a part amount of the hydrogenation product is fed back as diluent before the hydrogenation reactor, the metering of the base is advantageously carried out into the return stream before the latter is mixed with the aldehyde to be hydrogenated. In this way, local concentration maxima of the base during contact with the aldehyde, which can result in undesired aldolization, are avoided. If a reactor cascade is used, the requisite amount of base can be passed into the first reactor of the cascade together with the hydrogenation feed; however, it is also possible to meter the base separately into each individual reactor of the cascade. The total amount of base is preferably fed into the first reactor of the cascade together with the hydrogenation feed.
The hydrogenation catalyst used is one of the catalysts usually used for the hydrogenation of aldehydes to alcohols. The type of catalyst used is not a subject-matter of the present invention; the advantageous effects achieved by the process according to the invention are generally independent of the type of hydrogenation catalyst used. Accordingly, a multiplicity of hydrogenation catalysts can be used in the process according to the invention, for example metal-containing supported catalysts with metals from sub-group(s) I, VII and/or VIII of the Periodic Table as catalytically active components, in particular supported catalysts with rhenium, platinum, palladium, rhodium and/or ruthenium as catalytically active components and support materials such as aluminum oxide, titanium dioxide, silicon dioxide, zirconium dioxide or barium sulfate; or precipitation catalysts comprising at least one element from sub-group(s) I, VI, VII and/or VIII of the Periodic Table, for example catalysts as described in DE-A 32 28 881, DE-A 26 28 987 and DE-A 24 45 303. The catalysts are preferably in granular form and generally have a particle size of from 3 to 10 mm. The catalysts may be arranged in one or more beds in the reactor. Different catalysts can be used in the various beds of a reactor or in the various reactors of a reactor cascade.
The hydrogen-containing gas preferably comprises more than 80 mol % of hydrogen; in particular, it essentially consists of hydrogen. The hydrogen-containing gas can be passed over the hydrogenation catalyst bed in cocurrent or countercurrent to the hydrogenation feed. It is preferably passed in cocurrent. The amount of hydrogen-containing gas fed in is advantageously such that from 1.0 to 1.15 times the stoichiometrically necessary amount of hydrogen is available.
The hydrogenation of the alcohols can be carried out under conditions which are conventional per se. In general, elevated temperatures, for example from 100 to 300xc2x0 C., preferably from 120 to 250xc2x0 C. and in particular from 130 to 200xc2x0 C., and pressures of from 1 to 700 bar, preferably from 5 to 300 bar and particularly preferably from 30 to 50 bar, are set. The catalysts are generally loaded with from 0.01 to 2, preferably with from 0.1 to 1 and in particular with from 0.2 to 0.5 l of aldehyde/l of catalyst per hour. The addition of water to the hydrogenation feed is possible in the process according to the invention, but is not necessary.
The alcohols are generally worked up by distillation by methods known per se.
The aldehydes to be hydrogenated are preferably aliphatic C3- to C20-, in particular C3- to C15-aldehydes, which may be straight-chain or branched and may additionally contain double bonds in the molecule. The aldehydes which can be employed are not subject to any restrictions in principle. Suitable aldehydes which are of particular economic importance are, for example, propanal, n-butanal, isobutyraldehyde, hexanal, ethylhexanal, ethylhexenal, nonenal, nonanal, decanal, decenal and the hydroformylation products of trimeric and tetrameric propylene and dimeric and trimeric butene.
The process according to the invention has a number of advantages:
The acetal and ether formation side reactions which occur during hydrogenation of aldehydes in the liquid phase are greatly suppressed. The addition of water to the hydrogenation feed which was customary hitherto can be reduced or omitted completely. This enables the energy consumption in the subsequent distillation of the hydrogenation product to be significantly reduced, since the water is generally distilled off at the top of the column. The hydrogenation temperature can be increased without the fear of an increase in side reactions. This enables the space-time yield to be increased; for example, an increase in the hydrogenation temperature from 140 to 150xc2x0 C. in the hydrogenation of butanal allows the catalyst loading with butanal to be increased by 25% for the same butanol yield. An increase in ether formation on increasing the hydrogenation temperature, which is to be expected without addition of base, is not observed. The increase in the hydrogenation temperature liberates the heat of hydrogenation at a higher temperature level and can be utilized for generating steam at 4 bar, for example in the integrated heat system of the hydrogenation plant. This results in a considerable saving of energy.
The invention is illustrated in greater detail by the examples below.